1. Field of the Invention
The present invention relates generally to an adaptive antenna directivity control method and a system therefore in a mobile communication system applied a code division multiple access (CDMA) system or so forth. More particularly, the invention relates to an adaptive antenna directivity control method and a system therefore, in which amplitudes and a phases of transmission signals and reception signals of respective of a plurality of antenna elements are varied to form a combined radiation pattern for a particular orientation and to cancel output signals for other orientation, for forming a directivity.
2. Description of the Related Art
Conventionally, in radio communication, when a plurality of communication terminals perform communication simultaneously particularly at the same or close frequencies, communication jamming due to mutual interference, such as intermodulation or so forth, can be caused frequently. Therefore, multiplexed communication system permitting multiple simultaneous communication, such as known CDMA system, A frequency division multiple access (FDMA) and a time division multiple access (TDMA) have be employed. Even in such radio communication, it is still possible to cause mutual interference by a large number of communications in a limited frequency band and whereby to limit use efficiency of the frequency. On the other hand, it is also possible to cause interference due to adverse affect of a multiplexed wave propagation path (multipath) in metropolitan where many reflecting bodies, such as buildings, are present.
The CDMA system distinguishes each communication terminal from others by assigning an orthogonal code which has high auto-correlation and low cross-correlatioin. Thus, all of the terminals are performing communication using the same frequency. Particularly, in a mobile communication system, such as in a digital cellular mobile telephone system, propagation characteristics can be varied abruptly according to movement of the mobile terminal. In such wave propagating condition, orthogonality of the code can be broken to possibly cause degradation of communication quality due to mutual interference of communication. Therefore, in the CDMA system, a transmission power control for maintaining interference at respective terminals uniform, RAKE reception for effectively use a plurality of multipath waves having different delay periods, path capture and so forth according to IS-95 standard or so on, are performed.
In such mobile communication system according to the CDMA system, employment of an adaptive antenna has been attracting attention for the purpose of improvement of communication quality and improvement of use efficiency of the frequency. The adaptive antenna is constructed by regularly arraying a plurality of antenna elements for forming a special filter by appropriate weighting of amplitude and phase components for reception signals of different amplitudes and phases through respective antenna elements, in reception. In transmission, similarly to reception, by appropriate weighting of amplitude and phase components, arbitrary directivity characteristics (beam pattern and occasionally simply referred to as directivity) is established in antenna radiation.
As a result, in the mobile communication system, to which the CDMA system is applied, by use of the adaptive antenna, communication of a plurality of mobile terminals at the same frequency can be spatially separated to reduce mutual interference. Thus, use efficiency of the frequency can be improved.
In case of the cellular mobile communication system applied the CDMA system, associating with movement of the mobile terminal (occasionally referred to as mobile station), switching between cell base stations (occasionally referred to as base station) to establish radio channel connection is performed. Namely, so-called handoff is performed. In the cellular mobile communication system performing handoff, in particular the cellular mobile communication system according to the CDMA system, all base stations and mobile stations performs communication by spread spectrum using the same frequencies in respective of up-link and down-link. Thus, the mobile station can simultaneously communicate with both of the base station connected before handoff and the base station to be newly connected by handoff before the mobile station switches the base station. Namely, soft handoff can be performed.
In the mobile communication system employing the adaptive antenna which performs directivity control set forth above, in handoff of the CDMA system, directivity to the base station connected between handoff can be established stably and satisfactorily follow to the movement of the mobile station, however, sharp directivity (the beam pattern having small half value angle) to the base station to be connected cannot be established initially since orientation (direction/angle) of the mobile station cannot be determined. Accordingly, all orientation is retrieved at once to gradually predict the orientation of the mobile station to finally establish sharp directivity for the mobile station.
FIG. 15 is an illustration for explaining initial directivity control in the up-link in the base station to be connected by handoff to the mobile station when the conventional adaptive antenna is used.
In FIG. 15, the mobile station 3 is initially located in a cell area 5 (service area) where only transmission wave of the base station 1 reaches, and thus establishes radio channel connection with only base station 1. Thereafter, the mobile station 3 moves toward the base station 2 to be located near a cell area 6 (service area) of the base station 2. Then, soft handoff is initiated.
FIG. 15 shows directivities of respective up-links of the base station 1 and the base station 2 immediately after initiation of soft handoff in the mobile station 3. The up-link of the base station 1 as the base station connected before handoff appropriately follows to moving position of the mobile station and thus establishes sharp directivity (beam pattern 7).
In contrast to this, the up-link of the base station 2 establishes initial directivity (area 9) for retrieving all of orientations or only particular orientation (sector) since the position of the mobile station cannot be determined. Accordingly, the directivity (area 9) of the up-link of the base station 2 is the same area as the cell area 6 of the base station 2.
Broad (beam pattern having wide xe2x88x923 dB gain position) of the directivity (area 9) of the up-link of the base station 2 should increase possibility of mutual interference to receive a transmission wave of the up-link from another mobile station 4 located at the position of the orientation where the mobile station is not present. Furthermore, as set forth above, from the condition of omnidirectional retrieval, adaptive antenna directivity control has been initiated to take a significant period for restoring the normal state of control.
FIG. 16 is a sequence chart between a supervisory station, the base station and the mobile station in the case where the conventional adaptive antenna directivity control set forth above.
FIG. 16 shows a sequence including a supervisory station upon initiating soft handoff of the mobile station 3 from the base station 1 to the base station 2. The mobile station regularly notifies a received pilot channel strength. The notice from the mobile station is notified to the supervisory station via the base station 1. In the supervisory station, judgment is made whether the mobile station 3 can handoff with the base station other than the base station 1 on the basis of the content of notice. (xe2x80x9cHxe2x80x9d in FIG. 16).
Here, if there is no base station to be an object of handoff, no event occurs. On the other hand, if there is the base station to handoff (base station 2 in FIG. 15) (xe2x80x9cIxe2x80x9d in FIG. 16), the supervisory station commands for the base station 2 to assign communication channel for the mobile station 3 (xe2x80x9cJxe2x80x9d in FIG. 16). According to the command from the supervisory station, the base station 2 assigns the communication channel, and in conjunction therewith, sends a response indicative of completion of channel assignment to the supervisory station (xe2x80x9cKxe2x80x9d in FIG. 16).
Normally, at this stage, the base station 2 starts transmission through a communication channel of the down-link, and is enabled to receive communication through the up-link from the mobile station 3. However, as set forth above, at this stage, the directivity of the up-link is established for the predetermined wide area. On the other hand, the supervisory station receiving the response from the base station 2 recognized completion of preparation of communication between the base station 2 and the mobile station 3 to command initiation of soft handoff between the mobile station 3 and the base station 2 (xe2x80x9cLxe2x80x9d in FIG. 16).
The mobile station 3 receiving the command initiates reception of communication through the down-link from the base station 2. At the same time, completion of handoff is notified to the supervisory station via the base station. Here, at the stage of xe2x80x9cKxe2x80x9d in FIG. 16, since the base station 2 starts control from the state of the initial wide directivity and performs tracing of the mobile station 3, it takes a period longer than a period between xe2x80x9cExe2x80x9d to xe2x80x9cFxe2x80x9d in FIG. 5 which will be discussed later to restore the normal state control (xe2x80x9cMxe2x80x9d in FIG. 16). In other words, during this period, the base station receives communication from the mobile station 3 through the up-link and continues reception of the transmission wave from all orientation to cause mutual interference.
Similarly, even in the down-link, the base station connected before handoff can satisfactorily follow the mobile station with sufficiently stable directivity, whereas the base station to be connected by handoff has to perform transmission to entire sector area which is preliminarily determined upon installation of the base station.
FIG. 17 is an illustration for explaining an initial directivity control in the down-link of the base station to be connected by handoff when the conventional adaptive antenna directivity control set forth above is used.
FIG. 17 shows an example in which the pilot channel and the communication channel are to be controlled at mutually different directivities. On the other hand, movement of the mobile station 3 and the position thereof are the same as those shown in FIG. 15. An area 10 represents a directivity of a pilot channel in a certain sector (sector for the mobile station 3) in the base station 1. On the other hand, an area 11 represents a directivity of a pilot channel in a certain sector (sector for the mobile station 3). A beam pattern 12 represents a directivity of the down-link established by the base station 1 for the mobile station 3. On the other hand, a beam pattern 14 represents an initial directivity of the down-link established by the base station 2 for the mobile station 3.
The directivity of the down-link of the base station 1 which is connected before handoff, is established to satisfactorily follow the mobile station similarly to the case of the up-link, and is sharply established. In contrast to this, the directivity of the down-link of the base station 2 to be connected by handoff is to perform transmission through the down-link for omnidirection (cell area 6) since the orientation of the mobile station 3 is not known, or, in the alternative, to perform transmission to only particular sector area 1 by identifying the sector on the basis of information of the pilot channel of the base station 2, to which the mobile station establishes radio channel connection by the supervisory station and in accordance with the command of the supervisory station.
Therefore, when the base station 2 performs transmission through the down-link for the cell area 6, mutual interference can be caused in all of the mobile station other than the mobile station 3 located in the cell. On the other hand, when the base station 2 performs transmission through the down-link to the sector area 14, all of the mobile station other than the mobile station 3 located within the sector may cause mutual interference. Furthermore, similar to the case of the up-link, long period is required for restoring the normal state of control. It should be noted that the sequence of FIG. 16 is applicable for the down-link as is, discussion will be omitted. On the other hand, while discussion has been given for the case where the directivities of the pilot channel and the down-link are different, it is possible to control the directivities at the same directivity in certain system.
FIG. 18 is an illustration for explaining initial directivity control in the down-link of the base station to be connected by handoff to the mobile station in the case where the conventional adaptive antenna directivity control is used.
In FIG. 18, the directivities of the pilot channel and the communication channel are controlled to the same directivity. The beam pattern 15 represents the directivity of the pilot channel and the down-link of certain sector (sector for the mobile station 3) which is formed by the base station 1 for the mobile station 3. The beam pattern 17 represents the directivity of the pilot channel and the down-link of certain sector (sector for the mobile station 3) which is formed by the base station 2 for the mobile station 3.
It should be noted that a difference to FIG. 17 set forth above is only that the directivities of the pilot channel and the down-link of respective base stations 1 and 2 are the same, and others are basically the same. Accordingly, even in FIG. 18, the problem discussed in connection with FIG. 17 is encountered.
As set forth above, in the prior art, at every occasion of initiation of soft handoff in any mobile station, interference in the base station to be connected by handoff can be caused both in up-link and down-link. Furthermore, this problem is becoming more significant for substantial increase of occurrence of soft handoff since the recent mobile communication system, such as the cellular mobile telephone system has a tendency to narrow the cell area (service area) per the base station (formation of microcell), and in other words, channel capacity is reduced.
As prior art of handoff in this kind, there is Japanese Unexamined Patent Publication No. Heisei 10-70502, for xe2x80x9cDirectivity Controlled Antenna Devicexe2x80x9d. In this prior art, the antenna is controlled adaptively with predicting orientation of the mobile station for permitting efficient use of the frequency and transmission power.
On the other hand, Japanese Unexamined Patent Publication (PCT) No. Heisei 9-510595, for xe2x80x9cPhased Array Cellular Base Station with Improved Power Efficiency and Method Associated Theretoxe2x80x9d, facilitates use of an active phased array antenna by increasing power efficiency, reducing size of the cell area and by reducing radiation of spurious signal causing intermodulation in the base station.
In such prior art, the cell area of the base station tends to be reduced and frequency of occurrence of soft handoff is becoming higher. Therefore, interference in the base station to be connected by handoff tends to be caused frequently in both of the up-link and the down-link at every occurrence of initiation of soft handoff in any of the relevant mobile stations to lower communication quality at initiation of handoff. In such case, a problem of reduction of channel capacity can also be encountered. Therefore, even in the prior art disclosed in the above-identified publications, there has been still left a room for improvement.
The present invention has been worked out in view of the drawbacks in the prior art set forth above. It is an object of the present invention to provide an adaptive antenna directivity control method and a system therefore, which can reduce interference upon initiation of handoff in an up-link and/or a down-link to improve communication quality upon initiation of handoff, and can permit quick restoration of normal state of directivity control to result in increasing of channel capacity.
According to the first aspect of the present invention, an adaptive antenna directivity control method where a mobile station and base station to be connected with the mobile station in radio through an up-link and a down-link being governed by a supervisory station, respective transmission signal or reception signal of a plurality of antenna elements of the base station being varied in amplitude and phase for combining in a particular orientation for establishing a directivity by a radiation pattern by combining, comprises:
step of initiating soft handoff in the mobile station;
step of obtaining an installation position information from a first base station which is connected before handoff and directivity information of an up-link of the first base station relative to the mobile station;
step of predicting an orientation of the mobile station from own station by a second base station to be connected by handoff on the basis of the directivity information of the up-link; and
step of determining an initial directivity control parameter of the up-link corresponding to obtained orientation of the mobile station.
The directivity of the up-link to be initially set by the second base station may be controlled by determining a directivity control parameter so that the directivity of the up-link is narrower than a normal cell or sector area.
A directivity information of a down-link of the first base station may be used in place of the directivity information of the up-link of the first base station for predicting orientation of the mobile station by the second base station for determining initial directivity control parameter for the down-link.
The second base station may determine directivity control parameter of individual pilot channel for the mobile station together with the initial directivity control parameter of the down-link.
The directivity control parameter may be determined so that the directivity of the down-link which is used when the second base station transmits signal initially is narrower than normal sector area.
The initial directivity control parameter may be determined in the first base station upon initiation of soft handoff in the mobile station, by notifying the installation position information of the first base station and the directivity control parameter to the second base station through the supervisory station.
A position information of the mobile station relative to the first station may be notified to the second base station via the supervisory station in place of the installation position information of the first base station for determining the initial directivity control parameter in the second base station.
The installation position information of the first base station may be preliminarily notified to peripheral base stations from the supervisory station in replace of notifying to the second base station.
The orientation of the mobile station may be approximately derived in the second base station.
According to the second aspect of the present invention, an adaptive antenna directivity control system where a mobile station and base station to be connected with the mobile station in radio through an up-link and a down-link being governed by a supervisory station, respective transmission signal or reception signal of a plurality of antenna elements of the base station being varied in amplitude and phase for combining in a particular orientation for establishing a directivity by a radiation pattern by combining, comprises:
the mobile station including means for initiating soft handoff;
a second base station to be connected by handoff including means for predicting an orientation of the mobile station from own station on the basis of installation position information from a first base station connected before handoff and a directivity information of the up-link of the first base station, and determining an initial directivity control parameter of the up-link and/or a down-link.
The base station may comprise a wired circuit interface portion at least processing interface with a supervisory station, a radio receiving portion, a radio transmitting portion and a base station control and storage portion, and a directivity control parameter in the down-link in the radio receiving portion and/or the down-link in the radio transmitting portion is determined by control process of the base station control and storage portion.
The base station control and storage portion may be provided in or out of the radio receiving portion and/or the radio transmitting portion.